The present invention relates to servo systems, and more particularly to a voltage mode drive of a servo system to reduce drive amplifier circuit complexity.
Classical designs for digital controllers for driving a servo system include frequency- response methods where the gain and phase characteristics of the system in response to frequency are plotted in Bode plots manually. Simplistically for stability the frequency gain response should be less than unity when the phase response is 180 degrees out-of-phase in a negative feedback system. Therefore based upon the Bode plots a compensation circuit is designed to modify the response of the system to achieve stability with the desired gain. More recently a state-space formulation, using computer aided design (CAD) packages for the computation of discrete equivalents, has been implemented. The differential equations that describe the system are used and, using the CAD packages such as the MATLAB.TM. linear algebra matrix manipulation program from The Mathworks of Natick, Mass., the classical transfer function of a continuous system is transformed to the state-space continuous description. The final control algorithm consists of a combination of the control law and an estimator, with the control law calculations being based on estimated states rather than actual states. The estimator may be an error estimator where the difference between a desired output and the corresponding measured output provides an error signal. Alternatively the estimator may be a "current" (in time) or predictor estimator, as that is referred to in the text books, where estimates are based on present measurements directly. Using the state-space formulation a system matrix is generated for determining the control signal as a function of estimated states, and an estimator matrix is generated for determining the current estimates from prior estimates as a function of the control signal and a feedback signal based upon a measured state. These various designs are described in more detail in Chapters 5 and 6 of "Digital Control of Dynamic Systems" by Franklin, Powell and Workman, 2nd edition, published by Addison-Wesley Publishing Company, incorporated herein by reference.
Conventionally the state-space formulations being implemented generate a current (as in electrical current) control signal to drive the motor. The current control signal is applied through a transconductance amplifier to the windings of the motor. Transconductance power amplifiers, such as that disclosed in U.S. Pat. No. 5,296,792 issued Mar. 22, 1994 to David L. Knierim entitled "Bidirectional Chopper Transconductance Amplifier", incorporated herein by reference, have been traditionally used in electro-mechanical servo systems to drive a servo motor. The servo motor's torque is proportional to its current, not the applied voltage, and a transconductance, or current source, amplifier allows the servo designer to control or program the current directly. There is a tradeoff in this choice, however. Current source amplifiers are usually more complex and expensive than voltage source amplifiers because of the, costly current sense circuitry needed in the transconductance amplifier.
What is desired is a drive mode for a servo system that simplifies the hardware circuitry, especially the drive amplifier circuitry, required for the servo system.